Field of the Invention
The present invention relates to an EL panel (electroluminescent device) used in illumination devices, decorative illuminations, light sources for signs, and the like, and a display device using the EL panel.
Background Art
Generally, an EL element has a structure in which a hole injection layer, a hole transport layer, an interlayer layer, a light-emitting layer, an electron transport layer, and an electron injection layer are interposed between an anode and a cathode on a light-transmissive substrate.
An exciton is formed by applying a DC voltage to the anode and the cathode of the above-described structure, and then by injecting electrons and holes to the light-emitting layer for re-coupling, and light is emitted by discharging light when the exciton is in deactivation.
Conventionally, when a light beam exiting from a light-emitting layer has been emitted from a light-transmissive substrate of the light-emitting face side in such an EL element, part of the light beam has been totally reflected on the surface of the light-transmissive substrate of an observance side, and accordingly, a problem has occurred in that the amount of light to be extracted to the outside is lost.
It is believed that the light extraction efficiency of this case is generally approximately 20%.
For this reason, there is a problem in that as the demand for display or illumination with high luminance becomes greater, it is necessary to increase the amount of input power.
In addition, in this case, since a large amount of current flows in the device, a burden on the device increases, resulting in a reduction in luminance and shortened product life, and thereby the reliability of the device deteriorates.
Thus, for the purpose of enhancing the light extraction efficiency of light exiting from an EL element, an EL panel has been proposed which extracts light rays, which would be totally reflected by the surface of the light-transmissive substrate and become light loss, to the outside by forming fine unevenness on the surface of an observer side of the light-transmissive substrate of the light-emitting face side.
In order to enhance the light extraction efficiency as described above, for example, a micro-lens array 24 can be used, in which a plurality of micro-lens elements are arranged in a flat manner on one surface of a light-transmissive substrate 20 proposed in Japanese Unexamined Patent Application, First Publication No. 2002-260845, as shown in FIG. 17.
However, when the above-described micro-lens array 24 is used in an EL element, when a heat transfer body such as a finger is in contact with the micro-lens array 24, the EL element becomes dark at the contact point, whereby irregular luminance is shown in a bright area and a dark area in the in-plane of the EL element.
Since the area contacted by the heat transfer body continues to be dark for approximately several minutes to dozens of minutes, a shadow of illumination such as a standing light or a wall-mounted illumination is generated, which hinders practical observation of a light-illuminated object.
In the above-described conventional technique, when a heat transfer body such as a finger or the like is in contact with the surface of the EL element, heat on the contact part of the EL element is lost, changing the light emission efficiency of the EL element, whereby the part becomes dark.
Accordingly, there are a part where the finger is in contact with (dark part) and a part where the finger does not contact (bright part), and irregularity of luminance occurs in the in-plane of the EL element.
Particularly, in the case of an illumination device, it is easy for a finger or the like to contact the device, in terms of a practical aspect, whereby irregularity of luminance also easily occurs.